Stabilization of the spine for various conditions, including degenerative disc disease, scoliosis, spondylolithises and spinal stenosis often require attaching implants to the spine and then securing the implants to spinal rods. Such spinal fixation devices can immobilize the vertebrae and can alter the alignment of the spine over a large number of vertebrae by means of connecting at least one elongate rod to the sequence of selected vertebrae. Such rods can span a large number of vertebrae, such as three or four. However, the spine anatomy rarely allows for three or more implants to be directly in line. In order to allow for this irregularity, the rod must be contoured to the coronal plane. With anatomical curvature in the saggital plane found in the lumbar spine, the rod has to be contoured in both planes, requiring considerable effort and surgical time.
For example, the U.S. Pat. Nos. 5,554,157, issued Sep. 10, 1996, and 5,549,608 issued Aug. 27, 1996, both to Errico et al. disclose polyaxial locking screw and coupling element devices for use with rod fixation apparatus. The '157 patent discloses a coupling element including an interior axial passage having an interior surface which is inwardly curvate at the lower portion thereof such that it comprises a socket for polyaxially retaining a spherical head of a screw. The coupling element further includes a pair of vertically oriented opposing channels extending down from the top of the coupling element which define therebetween a rod receiving seat. The channel further provides the walls of the upper portion to a pair of upwardly extending members, each including an exterior threading disposed on the upper most portion thereof for receiving a locking nut. During the implantation of the assembly, the locking nut seats against the top of the rod which in turn seats on top of the screw head. The nut causes the rod to be locked between the nut and screw and the screw to be locked in the socket.
The '608 patent discloses a modification wherein a locking ring is disposed about the exterior of the lower portion of the coupling element and provides an inward force on an outwardly tapered portion upon downward translation thereof causing the interior chamber to crush lock a screw head therein to eliminate the polyaxial nature of the screw element coupling.
In both prior art embodiments, the locking mechanism locks both the rod and screw head simultaneously. No prior art patent allows for the spherical head of the screw to be locked at a desired angle prior to rod insertion. Likewise the only surface locking the rod in place is the surface between either the seat and a locking nut or the rod entrapped between a locking ring and the seat.
It would be desirable to increase the area of contact of the locking mechanism about the rod as this is a high stress site secured only by a friction fit. It would also be desirable to lock the screw head in place prior to fixation of the rod.